Is this the resonant EOM or a BBEOM driven with an external circuit? The driving requirements are different, if you overdrive you'll get a bunch of higher harmonics.
Maybe check the modulation depth, make sure it isn't too excessive. Beta=0.3 should be enough. Error signal Vpp = (where P0 is the incident power, and G is the PD + mixer gain. Just make sure beta it isn't excessive in producing significant higher order components.
You want to figure out if the 29.5 MHz harmonic feature is an electrical cross coupling or optical. Check your cable routing, are you getting any cross coupling pickup from collocated coaxial lines? Pretty sure we got the mode cleaner PDH demodulation stuff right, but check mixer is right power level with 4th order LP filter + 50 ohm termination on input side of the LP filter with a RF compatible T-junction.
Also, looking back at the design documents for the PMC, it looks like the nominal design frequency considered was ~30 MHz modulation. Have a look Evan's earlier calculations, especially with respect to HOM suppression. It looks like for 30 MHz the 60 MHz harmonic is in a relatively low transmission point for HOM. If the issues are optical artifacts transmitted it might be HOM... you can always up the frequency to something closer to 30 MHz.
Take all of the above with a grain of salt. Consider how much time to sink into it based on whether you think its actually driving a noise source in the system. Eliminating this 14.75 MHz harmonic may be in the basket of diminishing returns for your time. The 36 MHz and 37 Mhz AM components are a much bigger concern for FSS related coupling of RIN into frequency noise.
I hooked up an HP8560E to RF port of the South FSS Refl RFPD. I unlocked laser from the cavity and brought it to a place which seemed far away from any mode of the cavity.
I was hoping to see a peak at 37 MHz which is PDH modulation frequency of FSS on this path. To my surprise, signal level was ~-80 dBm only at 37 MHz. On zooming out, I saw a 29.5 MHz peak instead of about ~-55dBm. @9.5 MHz is second harmonic of the modulation frequency for South PMC PDH loop. This seemed weird as sidebands of EOM before PMC should not reach after PMC. So I have taken noted some preliminary numbers for future. This isn't complete analysis:
All data was taken with a span of 50 MHz. Note how on changing the frequency offset, I got a offset point where 37 MHz Peak almost disappears but the 29.5 MHz peak is present irrespective of the frequency. Need to investigate more.